Field of the Invention
The invention relates to a method for processing iron sulfate heptahydrate into iron sulfate monohydrate, and optionally into iron oxides and/or iron hydroxides.
Technological Background of the Invention
Large quantities of iron sulfate are obtained during production of titanium dioxide by the sulfate process, and also when pickling or etching ferrous metals. Iron sulfate is used in various sectors, e.g. as an additive in the animal feed and fertilizer industry, for water treatment in sewage treatment plants, as an agent for reducing chromate in cement, or as a raw material for iron oxide production.
Iron sulfate from technical processes is generally present in one of three states of hydration: heptahydrate, tetrahydrate and monohydrate. During the aforementioned production of titanium dioxide by the sulfate process, and when pickling or etching ferrous metals, the iron sulfate is primarily obtained in the form of heptahydrate. Iron sulfate heptahydrate has the highest water content of the three states of hydration and is difficult to store at temperatures in excess of 25° C., because it then melts in its own water of crystallization. Iron sulfate heptahydrate also has the lowest iron content of all hydrate forms, this resulting in a number of disadvantages in the target applications.
Consequently, various methods have been developed for further processing iron sulfate heptahydrate into lower states of hydration.
On an industrial scale, the heptahydrate is today predominantly converted into tetrahydrate or monohydrate by various kinds of thermal drying. The disadvantages of these methods is, however, their relatively high energy consumption, corresponding to at least the heat of evaporation of the water of crystallization to be removed.
Alternatively, a melting method in accordance with DE 1 230 409 can be used, where the iron sulfate heptahydrate is heated to up to 95° C., such that it melts in its water of crystallization, and iron sulfate monohydrate is precipitated and separated off. This method is more favorable than thermal drying in terms of energy, but roughly 25% by weight of the iron sulfate does not precipitate and remains in the mother liquor owing to the solubility behavior in the iron sulfate/water system.
DE 1 546 076 discloses a method for regenerating a spent etching solution, where iron sulfate monohydrate is added to the spent etching solution, whereupon iron sulfate heptahydrate is formed that is then separated off in solid form. The heptahydrate obtained can subsequently be heated to between 100° C. and 200° C., in order to again form iron sulfate monohydrate, which is returned to the process. DE 1 546 076 does not deal with adjusting the residual moisture content of the iron sulfate monohydrate produced.
There is thus a need for a method for the further processing of iron sulfate heptahydrate that at least overcomes the disadvantages of the prior art.